Conventionally, a pattern of a metal is formed by photolithography. An outline of the photolithography technique is described as follows. A resist solution is applied by spin-coating over a substrate on which a metal film is formed, and drying at a low temperature is performed so that excess solution is removed. Next, a photomask is caused to adhere to the substrate, and is irradiated with ultraviolet radiation. By so doing, regions having been irradiated with light (hereinafter referred to as irradiated regions) and regions not having been irradiated with light (hereinafter referred to as non-irradiated regions) are formed on a surface of the resist film. The irradiated regions and the non-irradiated regions have different resistances with respect to a resist developer. Next, the substrate is exposed to the resist developer so that the resist on the irradiated regions is removed (in the case where a positive-type resist is used) or the resist on the non-irradiated regions is removed (in the case where a negative-type resist is used). Thereafter, the density of the resist is increased by annealing the substrate at a high temperature, so that the resistance with respect to an etching solution is increased. Next, by exposing the substrate to the etching solution, the metal in the region not covered with the resist film is removed. Finally, the resist is removed by a resist peeling solution, whereby a metal pattern is formed. The photolithography method is an indispensable technique for forming semiconductor devices, with which patterns on sub-micron orders can be formed easily with accuracy at present.
The photolithography method is very useful as a metal patterning method, but it has several problems. The first problem is waste of materials, i.e., 90% or more of the resist solution is wasted in the resist application process. The second problem arises in that a large amount of an organic solvent, which possibly is harmful to the environment, has to be used in the developing and removing processes, and this is unfavorable from the viewpoint of the environmental protection. The third problem is in that in the case where various types of products are to be manufactured in small volumes, respectively, a costly photomask has to be produced for each of the product types, which increases the manufacturing costs. Still further, the photolithography method involves many steps, and if a method involving a smaller number of steps is developed, it will leads to a decrease in the manufacturing costs.
Recently, to solve the foregoing problems of the photolithography method, a metal pattern forming method employing the ink jet method, and that employing the microstamping method have been proposed. The following describes these methods.
(1) Metal Pattern Forming Method Employing the Ink Jet Method
This utilizes the techniques of the ink jet printer. The mechanism of the drawing by the ink jet printer (hereinafter referred to as ink jet method) is as follows: ink is jetted from a plurality of nozzle holes with a diameter of several tens μm each, formed in a nozzle plate, in amounts of several picoliters, respectively, toward a printing material such as paper, so that the jetted ink is arranged at a predetermined position on the printing material. To arrange the ink at predetermined positions on a recording medium, the ink is jetted by moving the nozzle plate and the printing material mechanically to control relative positions thereof. In principle, lines with widths on the submicron orders can be drawn by decreasing the size of the nozzle hole or devising the manner of how a liquid is jetted.
By jetting a metal paste by the ink jet method for drawing on a substrate, a metal pattern can be formed. By this method, the jetted material can be used without waste, which results in resource savings, and further, since the processes for developing and removing a resist are omitted, the use of a large amount of organic solvents can be avoided, which is favorable from the viewpoint of the environmental protection. Still further, since in the method employing the ink jet method, a liquid is jetted by using digital data prepared on computer so that a pattern is formed directly on a substrate, a costly photomask is unnecessary. Besides, since the exposing and developing processes can be omitted, small-volume manufacturing of a variety of products is possible at lower costs.
Alternatively, the process of drawing on a metal film with a resist solution by the ink jet method, and subsequently the etching and resist removing processes may be carried out, whereby a metal pattern can be formed also. By this method, only a minimum amount of a resist material is used, which results in resource savings. Further, since in the method employing the ink jet method, a liquid is jetted by using digital data prepared on computer so that a pattern is formed directly on a substrate, a costly photomask is unnecessary. Besides, since the exposing and developing processes can be omitted, small-volume manufacturing of a variety of products is enabled at lower costs.
Patent Documents 1 and 2 shown below propose methods for forming a metal wiring pattern in the following manner: a paste obtained by dispersing ultrafine metal particles with an average particle diameter of 1 nm to 10 nm in a resin composition is jetted by the ink jet method so that a metal wiring pattern is formed.
Patent Document 3 shown below proposes a method for forming a metal wiring in the following manner: a liquid metal in which particles having a sulfur compound adsorbed to their surfaces are dissolved is jetted by the ink jet method so that a metal wiring is formed.
Patent Document 4 shown below proposes a method for forming a circuit in the following manner: a circuit pattern is formed by jetting a metal particle ink by the ink jet method, the ink being obtained by dispersing metal particles with an average particle diameter of not more than 100 nm in water or an organic solvent, and thereafter, a treatment with heat or light is carried out so that polymers or a surfactant contained in the circuit pattern are discomposed or volatilized.
Patent Document 5 proposes a step of jetting a liquid resin through a jet head so as to form a desired pattern image on a substrate and hardening the formed pattern image with heat, an uncovered region corroding step of exposing the substrate having the hardened pattern image to an etching solution so that regions of the substrate not covered with the foregoing patterning resin are corroded, and a step of removing the pattern image that is no longer necessary.
Patent Document 6 shown below proposes a method for forming a pattern in the following manner: an ink that is solid in normal temperature and molten with heat is jetted in a molten state by the ink jet method so that a pattern is drawn on a conductive layer, and the conductive layer is etched so that the pattern is formed.
(2) Metal Pattern Forming Method Employing the Microcontact Printing Method
FIGS. 10A to 10E are schematic views showing steps of a method for forming a metal pattern by the microcontact printing method. A stamp 101 formed with a silicone resin is impregnated with an ethanol solution containing an alkane thiol (in the drawings, hexadecane thiol, CH3(CH2)15SH, is used), and the stamp 101 is pressed against a surface of a metal film 103 of gold, copper, or the like (FIG. 10A). By so doing, a pattern 105 of a monomolecular film of the alkane thiol is formed on the metal surface (FIG. 10B). Then, the metal is exposed to an etching solution so that a metal pattern 106 is formed (FIG. 10C). Thereafter, the monomolecular film is removed by using ozone or applying heat (FIG. 10D) (Non-Patent Document 1 below). 104 denotes a substrate. An alkane thiol has a property of being bound with a metal via SH by stamping and forming a monomolecular film 105 with a thickness of 1 nm to 3 nm (FIG. 10E). The monomolecular film, filled with molecules densely, does not allow the etching solution to permeate therethrough, and therefore, functions as a resist film for a metal. 102 denotes stamp regions impregnated with the alkane thiol, and 105 denotes the monomolecular film of the alkane thiol adsorbed to the metal film. It is also shown that the method enables the formation of a gold pattern with a width on a submicron order (Non-Patent Document 2 below). The stamping method requires a minimum amount of an alkane thiol as compared with the spin coating method, which results in resource savings. Further, since the exposing and developing steps are omitted, the number of steps decreases as compared with the photolithography method, which leads to the reduction of manufacturing costs. Still further, since the alkane thiol monomolecular film can be removed easily by a heat treatment at 100° C. or a higher temperature or by applying ozone, the use of a toxic organic solvent for removing a resist can be omitted, which decreases environmental loads.
The methods of Patent Documents 1 to 4 in which a metal paste is jetted by the ink jet method for drawing solve the problems of the photolithography method, but since the metal paste in the state of just being drawn has only a low conductivity, it is necessary to increase the conductivity by annealing. To obtain a conductivity at the same level as that of a metal used in the photolithography method, ideally the annealing at a temperature near the melting point of the metal is carried out. However, at such a temperature, a normal substrate is denatured due to heat. Accordingly, it is necessary to anneal the substrate at as low a temperature as possible. Particularly, in the case where the substrate is a printed substrate made of a resin, the annealing temperature preferably is not higher than 200° C. Therefore, the metal pattern formed by the ink jet method has a low conductivity, as compared with that formed by the normal photography method. Particularly in circuit wiring, a decrease in the conductivity is a significant problem since it leads to deterioration of the device performance.
In the cases of the methods of Patent Documents 5 and 6 in which a resist solution is jetted by the ink jet method, since a metal film used in the conventional photolithography method is used, a decrease in the conductivity does not occur, unlike the case where a metal paste is used. However, in the case where a resist solution is jetted by the ink jet method for drawing on a substrate, the jetted solution 111 spreads on the substrate 112 as shown in FIGS. 11A to 11C. 113 denotes the solution upon being brought into contact with the substrate, and 114 denotes the solution in a spreading state. Here, the spreading of the solution refers to a phenomenon in which an area in which the solution and the substrate are in contact with each other increases significantly as compared with an area of a cross section of the solution when being jetted out of the nozzle holes (4 πR2 where R represents a diameter of the solution). Depending on wettability of the substrate and properties of the jetted solution, the contact area sometimes increases to 10000 or more times the area of the cross section of the jetted solution drop. In the case where a solution is jetted in a region 121 as shown in FIG. 12A, if the solution does not spread, it is possible to form an accurate pattern 122 of the solution as shown in FIG. 12B, but if the solution spreads, only a blurred, inaccurate pattern as shown in FIG. 12C is formed. 123 denotes a region in which a pattern should be formed, and 124 denotes an actually formed pattern of the solution.
According to a method of Non-Patent Document 1 employing the microcontact printing method, a monomolecular film is formed only in a portion in which a stamp is in contact, and therefore, the obtained pattern rarely blurs. However, a stamp has to be prepared for each pattern, and hence, this method is not suitable for the manufacturing of a variety of products in small volumes.
Patent Document 1: JP 2002-299833 A
Patent Document 2: JP 2002-324966 A
Patent Document 3: JP10(1998)-204350 A
Patent Document 4: JP 2002-13487 A
Patent Document 5: JP 3245410 B
Patent Document 6: JP 2000-340928 A
Non-Patent Document 1: Applied Physics Vol. 63, p.4, 1993
Non-Patent Document 2: Synthetic Metals Vol. 115, p. 5, 2000